In web-fed printing presses, a continuous web of paper is printed with repeating images. These images have a certain non-printed space due to inherent features of locking a plate and a blanket onto the press cylinders. It is therefore an advantage in the cutting device to have an ability to trim off a narrow transverse strip of paper corresponding to the non-print area of the press. It is also desirable to produce a finished and trimmed product in a finishing line wherein a transverse strip of paper must be cut and removed from a continuous web to produce the desired product size.
There are several devices currently available to cut and remove such strips. One such device is described in U.S. Pat. No. 4,409,870 to Rynik et al. wherein a pair of knives mounted on a hollow rotating cylinder act against a stationary knife to cut the paper. As the paired rotary knives are close to each other, they produce a thin strip of paper which is sucked somewhat radially into the cylinder by a draft produced by an exhaust blower. The cut strips travel through the cylinder bore axially to the exhaust connection. This type of device is quite convenient compared to others requiring several movable components to hold and dispose of the strip. However, the mechanism of Rynik et al. suffers from several drawbacks that has limited its use. The major problem lies in the fact that several openings around the cylinder communicate with a central bore to which the vacuum exhaust source is connected. And as the cylinder must be supported in bearings, the size of exhaust connection is limited by the size of the bearing journal. To produce an adequate velocity of flow for conveying the cut strips through such an exhaust connection, a very large vacuum blower is needed. However, as the crosssectional area of openings connected to the central bore is much larger than the size of exhaust connection at cylinder end, there is a large drop in vacuum inside the cylinder. In order to alleviate this, the Rynik patent requires the use of a stationary sleeve inside the cylinder to limit the cylinder openings communicating with central bore. In actual practice, a stationary sleeve inside a rotating cylinder poses several problems with cut strips jamming up in the clearance between sleeve and cylinder. Such a device is also ineffective for high speed operation due to the fact that the knife openings are exposed to vacuum for a time interval smaller than needed to accelerate and move the cut strip from the point of cut to the central bore. The other drawback of the prior art consists of a poor aerodynamic flow pattern inside the cylinder with pockets of stagnant or low air velocity, so that at higher speeds, the cut strips tend to fly out of the cylinder due to centrifugal force. Due to these limitations, the cutting devices in accordance with the above described prior art have been limited to low speed operations.
It is accordingly the object of my invention to provide a rotary knife cylinder with improved aerodynamic design to enable cutting and removal of the strips at higher speeds and requiring a smaller sized blower by efficient utilization of vacuum.
It is an object of my invention to provide an improved cutting cylinder with no stagnant pockets and a streamlined air flow pattern with uniformly increasing cross-sectional area of flow in the direction of flow. It is also an object of my invention to permit use of a smaller sized cylinder due to the improved air flow pattern or alternatively to reduce the evacuated area within the cylinder.
It is a further object of my invention to have the greatest velocity of air flow at the beginning of the transport of the removed strip with a gradual reduction in velocity toward the delivery end of the cylinder produced by the gradual streamlined increase in cross-sectional area in the direction of air flow within the cylinder as well as beyond the cylinder.
It is also an object of the invention to reduce or eliminate articulations and seals between, for example, a stationary baffle and a moving cylinder as required in the prior art.